Pickup trucks are often equipped with cab guards or racks which extend vertically from the front rail of the truck bed behind the rear window of the truck cab. Some examples of such cab guards, which are also known as headache racks, are shown in U.S. Pat. Nos. 4,867,497, 6,983,968 and 7,121,585. Such truck bed racks are usually made of metal and can provide additional support or tie down locations for items loaded in the truck bed, and can provide an additional barrier which protects the rear window of the cab. However, conventional metal racks can scratch or damage certain types of items loaded in the truck bed. Furthermore, the metal racks are susceptible to corrosion, particularly if the metal is exposed or scratched.